1. Field of the Invention
The invention relates generally to a circuit configuration and method of manufacture of a transient voltage suppressor (TVS). More particularly, this invention relates to an improved circuit configuration and method of manufacture of a vertical transient voltage suppressor (VTVS) with EMI filter.
2. Description of the Relevant Art
The transient voltage suppressors (TVS) are commonly applied for protecting integrated circuits from damages due to the inadvertent occurrence of an over voltage imposed onto the integrated circuit. An integrated circuit is designed to operate over a normal range of voltages. However, in situations such as electrostatic discharge (ESD), electrical fast transients and lightning, an unexpected and an uncontrollable high voltage may accidentally strike onto the circuit. The TVS devices are required to serve the protection functions to circumvent the damages that are likely to occur to the integrated circuits when such over voltage conditions occur. As increasing number of devices are implemented with the integrated circuits that are vulnerable to over voltage damages, demands for TVS protection are also increased. Exemplary applications of TVS can be found in the USB power and data line protection, Digital video interface, high speed Ethernet, Notebook computers, monitors and flat panel displays.
FIG. 1A-1 shows a typical commercially available multi-channel TVS array 10. There are two sets of steering diodes, i.e., diodes 15-H and 15-L and 20-H and 20-L respectively for each of the two input/output (I/Os) terminals I/O-1 and I/O-2. Furthermore, there is a Zener diode, i.e., diode 30, with a larger size to function as an avalanche diode from the high voltage terminal, i.e., terminal Vcc, to the ground voltage terminal, i.e., terminal Gnd. At a time when a positive voltage strikes on one of the I/O pads, the high side diodes 15-H and 20-H provide a forward bias and are clamped by the large Vcc-Gnd diodes, e.g., the Zener diode 30. The steering diodes 15-H and 15-L and 20-H and 20-L are designed with a small size to reduce the I/O capacitance and thereby reducing the insertion loss in high-speed lines such as fast Ethernet applications. FIG. 1A-2 shows the reverse current IR versus reverse blocking voltage characteristics of the Zener diode between the Vcc and the ground voltage of the TVS 10 shown in FIG. 1A-1. The reverse current IR as that shown in the diagram of FIG. 1A-2 represents a reverse current conducted through the Zener diode, i.e., between Vcc and GND. Here it is assumed that the reverse BV of each steering diode is higher than the reverse BV of the Zener diode. But note that at high currents when the Vcc to Gnd pad voltage is equal or higher than the summation of the reverse BV of the steering diodes then the current would also flow through all the two series steering diode paths. Since the Zener diode has higher resistance per unit area compared with BJT or SCR and BJT this is actually a disadvantage at higher currents because the steering diodes also have to be rugged in reverse conduction. In the case of the SCR+BJT the Zener clamp voltage is lower at higher currents and hence the steering diodes paths will not conduct. The breakdown voltage of the Vcc-Gnd diode 30 and the steering diodes 15 and 20 should be greater than the operating voltage (Vrwm) so that these diodes only turn-on during the voltage transients. The problem with the Vcc-Gnd clamp diodes is that typically these diodes are very resistive in reverse blocking mode and require large area to reduce resistance. As shown in FIG. 1A-2, the high resistance leads to the increase of BV at high current. This is not desirable as high BV not only causes the break down of steering diodes as described above but also causes damage to the circuit the TVS device intends to protect. The requirement to have large diode size thus limits further miniaturization of a device when such TVS circuit is implemented.
For the purpose of reducing the size and surface areas occupied by the transient voltage suppressor (TVS) circuit, vertical TVS diodes are implemented as shown in FIG. 1B-1. The TVS is implemented with standard P substrate to N+ Zener avalanche diode with the cathode terminal formed on the top surface of a P-substrate doped with a N+ region below the cathode electrode. A metal layer is formed on the bottom of the substrate to function as the anode electrode. The P substrate usually has a resistivity of about 10-20 ohms-cm thus causes a high resistance of the diode. FIG. 1B-2 shows an equivalent circuit of a two channel vertical TVS diodes. The TVS diodes can also be integrated with an EMI filter as that shown in FIGS. 1C-1 and 1C-2. The vertical integrated configuration is similar to that of the vertical TVS diodes with an additional resistor interconnected between two vertical TVS diodes. Such vertical diode and EMI filter configurations as shown in FIGS. 1B-1 to 1C-2 suffer the disadvantage that there is great junction capacitance and has a poor clamping performance due to the high diode series resistance caused by the high resistivity of substrate.
Therefore, a need still exists in the fields of circuit design and device manufactures for providing a new and improved circuit configuration and manufacturing method to resolve the above-discussed difficulties. Specifically, a need still exists to provide new and improved TVS circuits that can provide low cost high density TVS and EMI filters for portable electronic devices.